Method for shirring casings

ABSTRACT

A method and apparatus for shirring sausage casings in which in the method the casing is moved in a direction of its longitudinal axis and while so moving has a force applied to shirr the casing which moves about the longitudinal axis of the casing. At the same time that the force is applied to shirr a force also is applied to oppose the movement of the casing. Also, the casing may be restrained against twisting about its axis upstream of the application of the shirring force. The apparatus includes rollers for forwarding a collapsed casing, an annular element adapted to rotate about the longitudinal axis of the casing to shirr the casing and a backing element adapted to hold the forward end of the casing in a fluid, tight manner and having a passage through which an inflating gas may pass to inflate the casing. The apparatus also may include grooved rollers adapted to hold the inflated casing against rotation prior to shirring and a lubrication device to apply a lubricant or a moistening agent to the casing prior to shirring. The annular element may be a sleeve having a helical projection on the cylindrical inner wall of the sleeve or a unit consisting of a plurality of holders for rotatably mounted wheels which are successively offset from one another so that in the rotation of the annular element they describe the same helical path.

This invention relates to a method and apparatus for shirring cylindrical casings, particularly sausage casings.

To enable a casing to be handled in a more rational manner when being filled, particularly with sausage meat, there is now an increasing need for casings that are shirred in the longitudinal direction. These shirred casings are required to possess properties in which the ratio of the length of the unshirred casing to that of the shirred casing (shirring ratio) is as great as possible and the shirred stick possesses high resistance to bending, is exactly straight, has the greatest possible inside diameter and runs off the stuffing horn easily and regularly. Furthermore, there can be no unsound areas in the casing.

The shirring of sausage casings has hitherto always been carried out by methods which in principle were based on a mechanizing of the shirring by hand. The shirring elements have therefore also always been moved in the direction of the longitudinal axis of the sausage casing. These known shirring methods have continued to be further refined with varying degrees of success and with varyingly increased complication of the technical means.

First, efforts were made to shirr sausage casings by holding portions of them by means of crimping fingers and pressing them against a backing means. Later, it was proposed to cause the crimping fingers to engage the sausage casing in such a way that the crimping force was not applied simultaneously to opposite parts of the casing. In this method the sausage casing was collapsed in the axial direction by means of crimping fingers positioned alternately at opposite sides of the casing. This resulted in a certain improvement since to some extent the folds in the sausage casing were hooked to one another. The shirring ratio was rather poor, however. The last-mentioned method is described in German Pat. No. 1,072,500. This patent also discloses the idea of applying the crimping force by crimping fingers each of which is moved in the direction of the axis of the sausage casing for effecting the crimping action, the fingers however being disposed one behind the other along a helical line on the sausage casing.

In order to shirr a sausage casing in a manner in which a substantially helical main fold is formed, it has also been proposed to carry out the shirring process with the aid of grooved shirring rollers which are caused to execute a slight eccentric movement. Two, three and four such eccentrically rotating shirring rollers were tried out with success (see German Pat. No. 1,192,072). For the purpose of obtaining a shirred stick having a helical main fold, it has also been proposed to operate with three crimping-finger wheels, the grooves between the teeth on each wheel being disposed in succession in a central position, in a position to the left thereof, and in a position to the right thereof: (see German Pat. No. 1,235,766).

Finally, it has been proposed to carry out the shirring process with crimping fingers which are inclined relatively to the longitudinal axis of the sausage casing. In this proposed arrangement, the crimping fingers are grooved and are applied to the sausage casing along an interrupted portion of a helical line. (See German Pat. No. 1,632,137, as published).

A feature common to all these known methods and the corresponding apparatus is that, during the actual crimping process, i.e. when the sausage casing is moved against the backing means or against the force opposing its movement, the crimping element (finger or portion of a roller) must on the one hand be moved in the axial direction and on the other must be retracted in the radial direction from the shirring zone in order to provide space for succeeding shirred material. As a result of this movement in the radial direction, the crimping fingers slide and scrape over the sausage casing. In this way risk of damage to the sausage casing arises. Numerous efforts have been made to overcome these disadvantages (see, for example, U.S. Pat. No. 2,583,654 and particularly FIG. 9 and the associated text in column 11), but these efforts have not resulted in the complete removal of the main weakness of these crimping processes.

The present invention provides in the first place a method of shirring cylindrical casings, particularly sausage casings, in which method the casing is moved in the direction of its longitudinal axis and shirred against a force opposing its movement, the force used for shirring being applied by at least one element which moves around the longitudinal axis of the casing. This method is extremely simple and reliable in operation, produces a shirred stick having an extremely uniform fold formation, avoids damage to the casing, can be applied to practically any diameter of casing, and can be carried out with apparatus of extremely simple construction.

In the preferred form of the method of the invention, the element or elements used for shirring is or are constantly in positive engagement with the casing during the shirring process.

In this method, the force used for shirring is constant in amount, and the force vector rotates around the longitudinal axis of the casing. Thus, by this method the possibility of carrying out the shirring process in a truly continuous manner is created for the first time. Shirred sticks in which the formation of the folds is extremely uniform are produced, and they possess a straightness not hitherto obtainable without additional measures.

The force required for shirring is in relation to the longitudinal axis of the casing continuously applied at the same point revolving around the axis. In this way it becomes possible to create absolutely constant and definite operating conditions, while applying a minimal stress to the sausage casing. This has not been possible in the known methods using crimping fingers, particularly since the axially moved crimping fingers were also required to execute a radial movement or a pivoting movement during the crimping process, and that at precisely the time when the strongest force was being applied to the sausage casing.

The method of the present invention can be performed by moving the casing to be shirred over a mandrel and up to a backing means, the mandrel being used for blowing in gas, and air in particular, i.e., for inflating the casing. This mandrel, however, performs the additional important functions of carrying the shirred casing, keeping it straight, and defining its inside diameter. Removal of the casing from the mandrel gives rise to some danger of damage to the casing, especially when very thin casings are being processed. To avoid this disadvantage, the mandrels are usually slightly tapered in the known manner and have a very smooth surface. It also appears desirable to provide internal lubrication between the mandrel and the casing, preferably using an oil for this purpose. The manufacture of mandrels having a very slight taper from the exact cylindrical form, and the other technical measures, are complicated and expensive.

A further subsidiary problem which can be solved by means of a preferred embodiment of the present invention is that of improving the method of the invention so as to eliminate the problems and disadvantages necessarily associated with the use of a shirring mandrel.

This is achieved when, in a method as above defined, the forward end of the casing is secured to a backing means to which the counter-force is applied, inflating gas is passed into the casing through the backing means and the casing being shirred is allowed to build up on the backing means to an extent such that the resulting shirred stick is unsupported except by a previously shirred portion of the stick. The stick is therefore not guided internally.

Surprisingly, it has been found that it is possible to carry out this method and also obtain a shirred casing which has a sufficiently large internal diameter over substantially the whole length of the sausage casing. A process of this kind could not be carried out by any of the known techniques, since in all of them the formation of the folds constituted an uncontrolled random procedure, and constant conditions only prevailed during very short periods in the shirring operation. In the method of the present invention, however, shirring takes place by folding the casing in an absolutely uniform and mathematically precise manner, so that a constant internal diameter of shirred casing is obtained depending on the outside diameter of the initial casing, the pitch of the shirring helix found on the shirred casing after extending it again, the shirring contour and the pressure in the casing to be shirred. In the case of a frustoconical shirring contour, the outside diameter Da and the inside diameter Di of the shirred casing, the pitch H of the helix on the extended shirred casing and the angle α between the frustoconical surface and the axis of the casing are related, with good approximation, by the following formula:

    Di = Da - H × sin α.

According to the present invention, the above-stated result can be achieved by a further preferred method wherein the pitch H of the extended shirred casing is so selected that it is less than the pitch Ho of a corresponding shirred casing of which the inside diameter, determined by the outside diameter of the original unshirred casing and the shirring contour, is equal to the outside diameter of the mandrel.

The pitch H is the pitch of the helical fold in the extended shirred casing. With the increase in the inside diameter of the shirred casing which is surprisingly obtainable by reducing the pitch H in relation to the pitch Ho, removal of the sausage casing from the shirring mandrel is made easier. In particular it is possible to work with a cylindrical mandrel instead of a slightly tapered one. The risk of damage to the shirred casing is reduced and the entire apparatus for carrying out the method and in particular for removing the shirred casing from the mandrel can be greatly simplified.

It has also been found that the reduction of the pitch H to a value less than the pitch Ho as above defined, is not only achievable by using a shirring element having this pitch. Surprisingly, it has also been found that it is possible to operate with a shirring element or elements in which the means for forming the impression in the casing have a pitch h which is greater than the pitch H required in the casing. In this preferred method, the said element or elements no longer simply slide in the recess impressed in the casing but also move along the casing transversely of this recess. Apart from the surprising result of being able to influence the inside diameter of the shirred casing in this way, the use of the method also leads to the element which makes the impression continuously overtaking the casing to some extent, as it rotates about the casing, and the casing is therefore stressed in a considerably more uniform manner than if the speed of advance of the casing were exactly the product of the pitch h of the helical projection on the shirring element (= pitch Ho of the helical impression in the extended shirred casing) and the speed of revolution f of the shirring element. Thus, in this case, the relationship v < h × f applies.

In addition, this method offers the considerable advantage that, without the need for changing the shirring element, the inside diameter Di of the shirred casing can be controlled by simply adjusting the ratio of the speed of advance v and the speed of revolution f, by adjustment of the pitch ##EQU1## Inside diameters of the shirred casing that are too close to the outside diameter are of course precluded, firstly because of the resultant loss of strength in the shirred casing and secondly because of the reduced shirring ratio.

In the case of a frusto-conical contour (see the equation given above), it is preferred according to the invention to select a pitch H such that the resultant inside diameter Di is greater than the outside diameter d of the mandrel. It will be clear from this relationship that for this purpose it is necessary to select a pitch H which is shorter than the pitch Ho. Thus, the approximate formula for this pitch Ho is: ##EQU2##

The invention also provides an apparatus for carrying out the method of the invention, said apparatus including means for moving a casing in the direction of its longitudinal axis, means for applying a force opposing the movement of the casing during shirring and shirring means connected to an annular element adapted to be rotated about the longitudinal axis of the casing.

The annular element preferably takes the form of a sleeve and the sleeve has for shirring a helical projection on the cylindrical inner wall of the sleeve. The helical projection may be a correspondingly shaped element fitting in a groove in the inner wall of the sleeve. The helical element may be made of a material having good frictional properties. Very good results have been obtained when the sleeve and the helical projection are integral.

In a further form of the apparatus of the invention, the annular element is a unit consisting of a plurality of holders for rotatably mounted wheels, which wheels are successively so offset from one another that they are arranged on a helical path.

An apparatus that has proved advantageous has downstream of the said helical projection an additional element adapted to be rotated about the casing in synchronism with the said annular element, said additional element also being arranged to engage the casing to effect shirring. The additional element is preferably a rotatably mounted wheel, the axis of rotation of the wheel being inclined to the longitudinal axis of the casing at an angle equal to the angle of pitch of the helical projection, so that the wheel, in use, also rolls in and along the recess formed in the casing by the said projection.

In an apparatus for performing the above-described shirring operation in which the shirred stick is in part selfsupporting, the means for applying the force opposing the movement of the casing during shirring includes a backing means adapted to hold the forward end of the casing in a fluid-tight manner and having a passage through which inflating gas can be supplied to the interior of the casing, the backing means being dimensioned to provide support for a short part only of a shirred stick of a length normally intended to be produced with the apparatus.

The apparatus described offers the considerable advantage that the shirred casing can be removed in a particularly simple manner. The casing, after shirring, has only to be removed from a relatively short holding means. The previous complicated arrangements incorporating a slightly tapered mandrel are completely dispensed with. Furthermore, the distance in the direction of the axis of the casing through which the shirred stick needs to be withdrawn is considerably shortened. In addition, the risk of damage to the shirred stick is much reduced.

According to the present invention, in a further apparatus, with which the problems associated with a normal shirring mandrel can be avoided, a hollow mandrel is provided to support the casing and the shirred stick, and a means for advancing the casing towards the shirring means and a means for rotating the latter are so mutually adjustable that the internal diameter of a shirred stick is somewhat greater than the external diameter of the mandrel.

In the case of an apparatus in which a shirred stick having a frusto-conical contour is produced, this means that the means for advancing the casing and the means for rotating the shirring means are so adjusted that the ratio ##EQU3## is smaller than Ho, Ho being obtained from the above equation, so that the following relationship applies: ##EQU4## wherein the symbols have the meanings stated above.

In a preferred apparatus in accordance with the invention, the means for rotating the shirring means and the means for advancing the casing are connected by a mechanical gear which, particularly when the speed of advance v is constant, allows for adjusting the speed of revolution f of the shirring means, and which incorporates a motor for driving the means for rotating the shirring means and the means for advancing the casing.

The present invention will be more clearly understood from the following detailed description made by way of nonlimiting example with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic side elevation of an apparatus for carrying out the method of the invention;

FIG. 2 is a similar view, partly in section, of part of the apparatus illustrated in FIG. 1, on a larger scale;

FIG. 3 is a similar view to FIG. 2 of part of a further embodiment;

FIG. 4 is a part end elevation, part section of a detail of the apparatus on a further enlarged scale;

FIG. 5 is a perspective view showing a shirring element with part cut away;

FIG. 6 shows various elevations of shirred casings each with part of a shirring element in section shown in the shirring position;

FIG. 7 shows, in end and side elevations respectively, two views of a further form of the shirring portion of the apparatus of the invention;

FIG. 8 is a diagrammatic side elevation of a further apparatus for carrying out the method of the invention;

FIG. 9 is an illustration of the geometry of a shirred casing, where the latter has a conical shirring contour;

FIG. 10 is a diagrammatic sectional illustration of a shirring element at two different stages of the shirring process; and

FIG. 11 illustrates the fold pattern on an extended and opened out shirred casing.

Referring to the drawings, in the apparatus illustrated in FIG. 1, a flattened sausage casing 1 is provided on a supply reel 2. Rollers 4 and 4' are driven by a motor 5 through a gear 6 and move the casing 1' over a mandrel 7 towards shirring and compressing means. Air is blown through the mandrel 7 in known manner in order to inflate the casing 1. The inflated casing 1' moves on towards a shirring unit 11 through two grooved rollers 9 and 9' driven by the motor 5 through a gear 8; the function of these rollers will be described in connection with FIG. 4. A lubricant can be applied to the casing 1' through nozzles 10. After the casing has passed through the unit 11, to be hereinafter described, it is compactly compressed. The shirring operation takes place with the casing being urged towards a movable backing means 13 which applies to the casing a constant counter-force in the direction opposite to that in which the casing is moved, this force being applied, for example, by a weight 16 suspended from a cord 14 passing over pulleys 15.

For a description of the actual shirring process, reference will now be made to FIG. 2. Inside a holder 17, a sleeve 18 is fixed to a V-belt pulley 19 and is rotatable on bearings 20. In its interior, the sleeve 18 has a helical inwardly extending projection 21, the configuration of which can be seen more clearly from FIG. 5. The V-belt pulley 19 is engaged by a drive belt 22 through which the sleeve 18 can be rotated.

The inflated casing 1' moves into engagement with the helical projection 21 as illustrated in FIG. 2. The peripheral speed of the rollers 4 and 4' and of the grooved rollers 9 and 9' is preferably so adjusted that it provides the exact speed for advancing the casing 1', which speed results from the speed of rotation of the sleeve 18 with the projection 21, and the pitch of the helix along which the projection 21 extends.

While the greater part of the helical projection 21 deforms the inflated casing 1' to a certain extent, that end 23 of the projection 21 that faces towards the backing means effects the actual shirring and compressing operation. This end 23 (see FIG. 5) presses the sausage casing, acted upon by the remaining part of the helical projection, in a direction opposite to that of the counter-force K applied by the backing means 13, this end 23 passing around the axis of the casing 1 during the shirring operation and constantly engaging the casing. Instead of a single projection 21, a two-start or multi-start helical projection can be provided. In the case of a two-start projection, two shirring elements offset from each other are provided.

A shirred casing 12 is produced in a single-stage operation by the method described; the casing can be easily removed from the mandrel 7 and has high resistance to bending and is extremely straight. The shirring ratio was found to be greater than 80:1.

A further form of the apparatus of the invention is illustrated in FIG. 3. This form differs from that illustrated in FIG. 2 in that, with it, the actual shirring and compression operation is no longer carried out by means of the end 23 of the helical projection 21 in the interior of the sleeve 18, but instead a small separate wheel 24 is provided for the purpose. A holder 25 for the wheel 24 is fixed to the driving pulley 19 surrounding the casing 1'. The wheel 24 is so fitted that it engages in the recess in the casing 1' that has been formed by the helical projection 21. Together with the pulley 19, the wheel 24 runs around the casing and compacts the casing into a succession of close folds, in which operation it overcomes the counter-force K.

A further form of construction is illustrated in FIG. 7. In this form of the apparatus of the invention, there is no spiral projection 21 in the sleeve 18. Instead, four holders 27 each accommodating a small wheel 28, are mounted on the pulley 19 and are offset from each other by 90° with respect to the longitudinal axis of the casing 1'. The axial length of these holders 27 increases by the same amount from wheel to wheel, i.e. at each quarter revolution. In this arrangement, the axes of the wheels 28 are so inclined that the wheels, in operation, impress a helical recess in the inflated casing 1' and then run in this recess. The wheel 28 nearest to the backing means 13 carries out the actual shirring and compressing operation by applying successive folds of the casing against either the backing means 13 or the already shirred portion 12 of the casing. The use of wheels 28 of this kind instead of a spiral projection 21 (FIG. 2) offers the advantage of exposing the casing to only a very slight amount of frictional load. The speed of the operation can therefore be greatly increased, and the danger of damage to the casing is reduced. The wheels 28 can, if required, also be driven through a planet gear, for example.

The mathematical axes of rotation of the wheels 28 are so inclined that each of them, at the point of contact, is disposed perpendicular to the tangent to the helix which they describe on the casing 1'. The distance between this axis of rotation and the axis of the casing is equal to the sum of the radii of the casing and of a wheel 28 minus the depth of the impression made in the casing 1' by a wheel 28.

Reference will now be made to FIGS. 2 and 4, for the purpose of describing the function of the grooved rollers 9 and 9'. The projection 21 engaging the casing 1' and, to some extent also, the wheels 28, tend to turn the casing 1' in their direction of rotation as they move around the axis of the casing. This tendency is intended to be counteracted by the driven rollers 9 and 9'. The inflated casing 1' moves into the grooves in the rollers 9 and 9'. This method of driving completely precludes undesirable twisting of the casing 1'.

FIG. 6 illustrates various examples of forms of a compressed shirring contour obtainable by the method of the invention. The particular purpose of these various forms is to impart high resistance to bending to the finished shirred casing and at the same time to obtain a good shirring ratio.

Apart from providing a solution to the initially stated problem, the method and apparatus of the invention enable the following advantages to be obtained. The possibility is offered of producing a finished shirred casing 12, e.g. a compressed sausage casing having a shirring ratio of 80:1 and above, in a single operation, i.e. without the need for a subsequent compressing stage. This operation can also be carried out in a manner causing no damage to the casing, and it results in a shirred casing which is completely free from defects despite the small wall-thickness of the casing, the products having a fold formation of a uniformity hitherto not known and possessing high resistance to bending.

The apparatus is extremely simple in construction and reliable in operation. One and the same apparatus can be designed for dealing with different diameters of casing, and the machine can be changed over for handling various diameters by simple manual means or even automatically. The method of the invention can be operated more rapidly and economically than any of the known methods.

The apparatus illustrated diagrammatically in FIG. 8 is substantially the same as that shown in FIG. 1. It does, however, differ therefrom in that at the commencement of the shirring operation, the beginning of the casing 1 is drawn on to a short tubular element 30 which projects from the backing means 13, and at this point the casing is gripped by two jaws 31 and 31' actuated by two compressed air cylinders 32 and 32' respectively. Compressed air is fed through an opening 33 into the interior of the casing 1 so that the latter is inflated over its length beginning at the point where the casing is in contact with the rollers 4 and 4'. The actual shirring process takes place, in the manner previously described, in the shirring unit 11.

It has thus been found that when shirring by this method, a mandrel can be dispensed with completely. In the method described, a shirred casing 12 is formed that is so uniform and precise in shape and straightness that even in the case of fairly long lengths of shirred casing, the shirring operation can be carried out by pressing the casing against self-supporting already shirred material 12. The short tubular element 30 is here used simply to anchor the casing. In this way a casing has been shirred to produce a stick that was 25 cm in length and had a shirring ratio of 80 and above, no subsequent compressing operation having been required. The self-supporting shirred length 12 of the casing had a very uniform circular interior and there were no isolated, inwardly projecting folds, so that a stuffing horn for filling the casing with sausage meat could be introduced without difficulty. In the example described, the inflated casing 1' had a diameter of approximately 18 mm. The inside diameter of the shirred casing was a little above 11.2 mm. The shirring element used had a pitch of 12 mm. The finished shirred casing can be removed from the backing means in an extremely simple manner. The casing has only to be pushed off the tube 30, with the aid of the jaws 31 and 31' for example, the casing being located quite loosely on the tube. Thus, it is not necessary to use the previous procedure, complicated as regards equipment and technique, for pushing the finished shirred casing 12 off a slightly tapered mandrel as illustrated in FIG. 1 and designated by the numeral 7.

FIG. 9 shows the geometry of a shirred casing having a frusto-conical shirring contour and produced by the method of the invention. The following relationship exists between the various geometrical values:

    Di = Da - H . sin α

herein:

Di is the inside diameter of the shirred casing,

Da the outside diameter of said casing,

H the pitch of the helical fold in the shirred casing when again extended, and

α the angle formed between the generatrix of the frusto-conical portion and the axis of rotation. This relationship is not quite exact, but provides a good approximation.

The important feature of the method of the invention by which a shirred casing is produced having an inside diameter greater than the outside diameter of the mandrel will now be explained by reference to FIGS. 9 to 11. The above-described method was carried out using seamless spun "Cellophane" (Regd. Trade Mark) casing material having a width of 27 mm when flat. A cylindrical mandrel having an outside diameter of 12 mm was used. The pitch h of the helical shirring element 21 was 20 mm. The angle of inclination of the frusto-conical portion (see FIG. 9) was approximately 30° to 40°.

First the shirring operation was carried out with the sausage casing being advanced at a speed v and the shirring element being rotated at a speed f, these speeds giving the relationship

    v = h × f.

The shirred casing so formed was firmly located on the mandrel and could only be removed therefrom by the use of considerable force. The pitch H (see in particular FIG. 10) was likewise 20 mm in this case.

The shirring operation was then carried out using the same material and the same mandrel. This time however, the speed of rotation f was changed so that the pitch H was 12 mm, the relationship v = H × f also applying in this case. Thus, the relationship with the pitch h of the shirring element was

    V< h × f.

The shirred casing obtained in this instance had an inside diameter Di of 12.2 mm. The casing could be easily pushed off the mandrel.

Reference will now be made to FIG. 10 for the purpose of further explaining this method of shirring called the "overtaking" method. The two successive positions differ by one revolution of the shirring element. In the upper chronologically first position a point, marked with a cross, on the helical element 21 might be in engagement with a point, marked with a circle, on the casing 1' to be shirred. After one revolution of the sleeve 18 the corresponding point, marked with a cross, on the helical element 21 that is now positioned at a place one complete turn further on in the direction of movement of the casing and is likewise marked with a cross, is no longer in engagement with the point on the casing 1' marked with a circle. It can be seen from this that the helical element 21 is no longer simply "cutting a screw-thread" in the recess in the casing 1' (the speed of revolution of the sleeve and the speed of advance of the casing no longer quite conform to the first-mentioned relationship v = h × f), but that the helical element 21 is also overtaking the casing. With this mode of operation, it has been found that the inside diameter of the shirred casing that is formed becomes greater than when the operation is carried out in accordance with the relationship v = h × f. Thus, it has become possible for the first time to operate with a cylindrical mandrel and yet to achieve easy stripping of shirred casings having a length of 38 cm. Thus both the hitherto considerable risk of damage to the shirred casing and the need for a tapered mandrel are eliminated. 

What we claim is:
 1. A method of shirring cylindrical casings particularly sausage casings, comprising moving the casing in the direction of its longitudinal axis and shirring against a force opposing its movement, the force used for shirring being applied concentrically of the axis of the casing by at least one element which moves about the longitudinal axis of the casing.
 2. The method of claim 1, wherein the element or elements used for shirring is or are constantly in positive engagement with the casing during the shirring process.
 3. The method of claim 1, wherein prior to the actual shirring operation, a recess in the form of a continuous helical groove is impressed in the casing.
 4. The method of claim 1, wherein prior to shirring, at a point a short distance upstream of the point at which the shirring force is applied, the casing is restrained against twisting about its axis.
 5. The method of claim 1, wherein the forward end of the casing is secured to a backing means to which the counter-force is applied, inflating gas is passed into the casing through the backing means and the casing being shirred is allowed to build up on the backing means to an extent that the resulting shirred stick is unsupported except by a previously shirred portion of the stick.
 6. The method of claim 1, wherein the pitch H of the extended shirred casing is so selected that it is less than the pitch Ho of a corresponding shirred casing of which the inside diameter is equal to the outside diameter of the mandrel.
 7. The method of claim 6, wherein, in the case of a frusto-conical contour of the shirred casing, the helical fold formed on the extended shirred casing has a pitch H which is less than Ho as derived from the relationship ##EQU5## wherein Da is the outside diameter of the shirred casing, d the outside diameter of the mandrel and α the angle of inclination of the generatrix of the frusto-conical portion with respect to the axis of the shirred casing.
 8. The method of claim 6, wherein a shirring element is used which impresses a helical recess in the casing, the means on this element for making the impression having a pitch h which is greater than the pitch H of the extended shirred casing. 